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#### ikumar

##### Newbie level 2
My DSL modem power cycles when the grid power goes off and switches over to my inverter/battery source. I tried connecting the modem adapter to a stabilizer, UPS both did'nt help. I'm wondering if I could add a capacitor to hold the modem up (for a second?) when the power source switches from grid to inverter. Please suggest capacitor type, should I be using a resister also.

simple way is to decide the value of the capacitor by a simple calculation. note down the switching time between the grid and UPS source, i.e. the time when the power goes off and the UPS starts delivering power. this time interval must be compensated by a capacitor, supplying the power. measure the approx resistance (or impedance) between the power terminals (+ and -) of the Modem. now T = RC . this implies, C =T/R. Hence the Capacitance in Farad = Time interval in seconds / measured resistance in ohms.

Use a capacitor with higher value than the value obtained from the formula, on the safer side. then make sure the rated voltage is at-least 2ice the operating voltage. u can add a diode before capacitor to avoid reverse flowing of current during discharge. !!

simple way is to decide the value of the capacitor by a simple calculation. note down the switching time between the grid and UPS source, i.e. the time when the power goes off and the UPS starts delivering power. this time interval must be compensated by a capacitor, supplying the power. measure the approx resistance (or impedance) between the power terminals (+ and -) of the Modem. now T = RC . this implies, C =T/R. Hence the Capacitance in Farad = Time interval in seconds / measured resistance in ohms.

Use a capacitor with higher value than the value obtained from the formula, on the safer side. then make sure the rated voltage is at-least 2ice the operating voltage. u can add a diode before capacitor to avoid reverse flowing of current during discharge. !!

Thanks kgram, I'm not sure how I could measure the switching time from grid to UPS source, but I'll start experimenting with a capacitor and add more of them in parallel and see if they could hold my modem up.

Suppose power drops out for a couple of AC cycles. To carry the modem through the gap, a capacitor will need to be about the same value as if you were constructing a linear type power supply for the same load.

Which is to say, on the order of 1000 uF or more, if we assume the modem uses maybe 250 mA.

It may work better to install a battery pack to serve as a continual UPS.

Caveats: (1) The batteries can be subject to a continuous miniscule trickle charge but they must be protected from overcharging.
(2) The pack must be adjusted to put out exactly the same volt level as the modem's normal power supply.

To solve the caveats will take some effort.

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